1. Field of the Invention
This invention generally relates to a linear motion guide unit, and, in particular, to a linear motion rolling contact guide unit capable of absorbing an error in alignment in the transverse direction. The present linear motion guide unit is particularly suitable for use in a drive unit of various machining tools, high precision processing apparatuses and testing apparatuses.
2. Description of the Prior Art
A linear motion guide unit for guiding the motion of a relatively large load over a relatively long distance is well know in the art. Typical prior art examples of such a heavy duty linear motion guide unit are shown in FIGS. 6 and 7.
FIG. 6 illustrates a heavy duty linear motion guide unit which includes a pair of rails B and C, which are fixedly attached to a bed A, a plurality (four in the illustrated example) of slider assemblies D and E, and a table F to which the slider assemblies D and E are fixedly attached. The rails B and C extend in parallel over a relatively long distance, and each of the rails B and C is formed with a pair of guide grooves H and I on its opposite side surfaces, which extend in parallel with the longitudinal axis of the rail. Each of the slider assemblies D and E is formed with a pair of endless circulating paths which are filled with a plurality of rolling members and each of which has a load path section where the rolling members are partly exposed to be partly brought into engagement with the corresponding guide groove H or I. As a result, a rolling contact is established between the rails B and C and the associated slider assemblies D and E, so that the table F can move linearly along the rails B and C back and forth as indicated by the double arrow G. Any desired object may be fixedly mounted on the table F.
With the structure shown in FIG. 6, in order to provide an accurate movement, the rolling contact between the rails B and C and the associated slider assemblies D and E is preloaded or set with substantially zero tolerance. However, since there is a limit in arranging the rails B and C in parallel, there is normally an error in the degree of parallel arrangement. Thus, if the rails B and C extend relatively over a long distance, the true distance between the two rails B and C vary or fluctuate, so that the sliding resistance varies as the table moves along the rails B and C. This is quite disadvantageous because of lack of smooth movement, possibility of excessive local wear and unreliability in operation. It is true that the problem could be relaxed if the rails B and C were aligned as accurately as possible. However, this would require excessive care and skills in mounting the rails B and C on the bed A. For example, it would be almost impossible to arrange the rails B and C on the bed A with an accuracy on the order of several tens of microns or less if the rails B and C are a few meters long.
FIG. 7 illustrates another typical prior art heavy duty guide unit. In this case, a pair of rails J and K is fixedly mounted on a base, and a table L is slidably mounted on the rails J and K with a plurality (six in the illustrated example) of endless circulating type bearing units M1 through M6 interposed therebetween. That is, four bearing units M1 through M4 are fixedly mounted on the left end portion of the table L such that they are brought into rolling contact with the respective four surfaces of the rail J. On the other hand, two bearing units M5 and M6 are fixedly mounted on the right end portion of the table L such that they are brought into rolling contact with the respective top and bottom surfaces of the rail K.
With this structure, even if there is an error in parallel alignment between the rails J and K, the bearing units M5 and M6 can move in the transverse direction indicated by the double arrow N to absorb such an error. On the other hand, the left end portion of the table L does not change the relative positional relationship with the rail J since the bearing units Ml and M3 are in rolling contact with the opposite side surfaces of the rail J under pressure. In this case, however, a relatively large number of endless circulating bearing units, which are relatively large in size, must be fixedly mounted on the table L and a relatively large number of lubricant paths must be formed in the table L. As a result, the overall structure tends to be large in size, which is diametrically opposite to the current trend for miniaturization of linear motion guide units. Besides, this structure does not allow to use existing typical linear motion rolling contact guide units and rails therefor.